Because of the intensity and precision of their radiation, lasers have many useful applications to the treatment of surfaces. For example, laser heat treating of metals has become a valuable industrial process since it provides a means for selectively hardening specific areas of a metal part. Lasers have also become valuable medical instruments. In dermatological applications, however, the laser has resisted widespread use due to problems such as variable depth penetration, nonuniform exposure, and consequent charring of tissue. Ideal skin resurfacing, for example, requires efficient tissue vaporization over usefully large areas, precise vaporization depth control, and the appropriate depth of residual thermal effects (about 50 to 150 .mu.m). To confine ablation and thermal coagulation to a thinlayer, it is necessary to use wavelengths that are easily absorbed in the superficial layer of tissue, for example the 10.6 .mu.m wavelength of a CO.sub.2 laser. Moreover, the laser energy must be delivered in a short time interval (less than 1 ms) in order to prevent thermal damage to surrounding tissue. Finally, the laser beam must have an energy density that is large enough (about 5 J/cm.sup.2) to vaporize tissue. Because of these numerous constraints, ideal skin resurfacing has not been possible in the past.
Continuous wave laser treatment for skin resurfacing often involves inadvertent thermal damage and subsequent scarring to healthy tissue. The use of pulsed lasers can reduce the possibility of thermal damage, and make lasers less hazardous, but thermal damage has not been eliminated and persists in discouraging the use of lasers. Coherent Lasers Inc. of Palo Alto, Calif. has recently introduced an improved pulsed surgical laser system that solves some of the previous difficulties by delivering higher energy pulses (500 mJ/pulse) with higher energy density, shorter duration, and an interpulse duration longer than the thermal relaxation time of tissue. This permits tissue ablation with less thermal damage to the surrounding tissue than caused by previous systems. This system, however, has some significant disadvantages. Since the surgical procedure for skin resurfacing involves evenly "painting" the treatment area, the task of uniformly treating a large surface of skin with a manually controlled laser delivery system is time-consuming and error-prone. Pulsed laser systems are also very expensive. Moreover, the laser beam itself has a nonuniform gaussian intensity profile, causing suboptimal ablation even for single craters, which are needed in hair transplantation techniques.
U.S. Pat. No. 5,411,502 issued May 2, 1995 to Zair discloses a system intended to produce uniform ablation of tissue through the use of automated scanning. As shown in FIG. 1, a continuous laser beam 20 is reflected off two rotating mirrors 22, 24 whose optical axes are tilted at angles with respect to their rotational axes, thus causing the beam to scan the surface uniformly in the pattern of a Lissajous FIG. 26. A refractive lens 28 is used to focus the beam. The scanning movement of the beam over the surface produces a short-duration local tissue interaction similar to that of a pulse. Because of the scanning, a large region is exposed. The exposure, however, is not completely uniform since a Lissajous figure is self-intersecting and is not space-filling. Moreover, the treatment even at a single point along the path is uneven because of the nonuniform intensity profile of the laser beam. In addition, the use of refractive optics introduces its own problems. Lenses limit the wavelengths that can be transmitted by the system and restrict the versatility of the device. Lenses also introduce chromatic aberration that causes a superimposed aiming beam to diverge from the invisible treatment beam.
Sharplan, Inc. of Allendale, N.J. manufactures a laser scanning system for dermatological applications, shown in FIG. 2. Using two microprocessor-controlled mirrors 30, 32 and a focusing lens 34, it directs a laser beam 36 at a constant velocity in a spiral pattern 38 over a circular area. The spiral path produces a more uniform exposure than the Lissajou path, but the exposure is still not optimally uniform. FIG. 3 illustrates the power distribution of the laser beam and the effect of scanning on the tissue. Because the gaussian power distribution 40 of the laser beam is not uniform, the tissue at the center of the spot receives more energy than that at the edges of the spot, resulting in undesired tissue effects 42. Although the spiral scanning pattern 44 helps to reduce these effects, it does not eliminate effects at the edges 46 of the scan or when the device is used to create single craters, as is required in certain applications such as hair transplantation. Moreover, since exposing tissue twice with the low-power edges of the beam is not equivalent to exposing once with the high-power center of the beam, the scan does not entirely eliminate imperfections due to the gaussian distribution of the laser spot. This system also has all the disadvantages mentioned earlier associated with lens-based optical systems because it uses refractive lenses to focus the laser beam.
U.S. Pat. No. 4,387,952 issued Jun. 14, 1983 to Slusher discloses a laser scanning system for heat-treating metals. The scanning and focusing of the laser beam are produced by two rotating concave mirrors tilted at small angles with respect to their axes of rotation, similar to the system shown in FIG. 1 except without the refractive lens. The mirrors are rotated in phase and in opposite directions resulting in a linear scanning pattern that produces a uniform delivery of laser energy to the surface. The rotation mechanism includes a precision timing drive with phase adjustment. Because this system uses reflective optics, it overcomes the disadvantages of lens-based optical systems. It does not, however, solve the problems due to the nonuniform intensity distribution of the beam and does not teach methods for scanning two-dimensional regions.
U.S. Pat. No. 5,128,509 issued Jul. 7, 1992 to the present inventor discloses a delivery system, shown in FIG. 4, which uses reflective optics to steer and focus a laser beam 48. The optical focusing is performed by a convex mirror 50 and a concave mirror 52 facing each other and aligned on a common optical axis 54. The laser beam passes through a small hole 56 in the center of the concave mirror and is reflected by the convex mirror back towards the concave mirror. The concave mirror reflects the beam forward to a focus 58 beyond the convex mirror. Because this system uses reflective optics, it is capable of delivering laser beams of a wide range of wavelengths and to a very small focus. Unlike systems using refractive optics, it can simultaneously deliver coincident far IR and visible beams. Moreover, because reflective optics do not exhibit chromatic aberration, it delivers the two beams to the same focal point. This system, however, does not provide a means for scanning to produce a uniform exposure over a large surface area.